There are a number of methods in which vibratory energy, such as ultrasound, is used to produce images of objects, such as in medical applications for imaging internal areas of patients for diagnostic purposes. An ultrasonic transducer array is used for both transmission and reception of ultrasonic pulses with an image produced in which the brightness of each pixel of the image is a function of the amplitude of the ultrasound reflected from the imaged object to the receiver, which in turn is determined by differences in characteristics or materials of the object being imaged.
A typical ultrasonic probe for medical applications comprises a transducer array made up of a multiplicity of piezoelectric elements, each element being sandwiched between a respective ground electrode and respective signal electrode. When an appropriate voltage pulse is applied, the piezoelectric element emits an ultrasonic pulse into the medium of interest, such as the body of a patient. Conversely, when an ultrasonic echo pulse strikes the piezoelectric element, the piezoelectric element produces a corresponding voltage across its electrodes.
The piezoelectric elements of a typical ultrasonic probe are arranged in an array such that by properly controlling relative time delays of the applied voltages on each element, the ultrasonic waves produced by the piezoelectric elements can be made to combine to produce a net ultrasonic wave focused at a selected point. This focal point can be moved on each successive transmitter firing, so that the transmitted beams can be scanned across the object without moving the probe.
Similar principles apply when the probe is employed to receive the reflected sound. The voltages produced at the transducer elements in the array are individually delayed in time and then summed together such that the net received signal or "beamsum" is dominated by the received sound reflected from a single receive focal point in the object. The individual pixels, when combined, provide an image of the object, such as a fetus or an internal organ of the human body.
Any noise or incoherent signals present in the beamsum signal detracts from the image quality through destructive interference. Therefore, various methods of filtering noise out of the received signal to enhance image presentation and imaging have been used or attempted. However, present filtering methods are not completely satisfactory. Signal-dependent noise such as speckle noise commonly observed in coherent imaging systems such as ultrasound systems for medical and industrial purposes and even in synthetic aperture radar and laser imaging--cannot be properly or adequately handled by conventional filtering techniques. On an ultrasound image, speckle noise visually appears similar to the familiar "snow" or noise spotting of television images provided by a home television receiver.
Conventional techniques for filtering additive noise in ultrasound imaging often fail if the noise is multiplicative or signal dependent. In many applications, particularly medical ultrasound imaging, loss of the true or information-containing signal as a result of the filtering operation is highly undesirable or unacceptable for diagnostic imaging. Noise filtering techniques based on Fourier analysis assume that the noise is dominant in the higher frequencies. Such assumption is often crude and inaccurate for various types of signals. Various attempts to remove speckle noise have not been satisfactory. Therefore, a key feature of denoising ultrasound signals is to retain important signal information while removing as much of the noise as possible to improve the signal-to-noise ratio.